Fuel cells have been proposed as a power source for many applications. One well known such fuel cell is the PEM (i.e., proton exchange membrane) fuel cell. PEM fuel cells include, in each cell thereof, a so-called "membrane-electrode-assembly" (hereafter MEA) comprising a thin (i.e., ca. 0.0015-0.007 inch), proton-conductive, polymeric, membrane-electrolyte having an anode electrode film (i.e., ca. 0.002 inch) formed on one face thereof, and a cathode electrode film (i.e., ca. 0.002 inch) formed on the opposite face thereof. Such membrane-electrolytes are well known in the art and are described in such U.S. Patents No. as U.S. Pat. Nos. 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, Volume 29 (1990) pages 367-387, inter alia. In general, such membrane-electrolytes are made from ion-exchange resins, and typically comprise a perfluoronated sulfonic acid polymer such as NAFION.TM. available from the E.I. DuPont de Nemours & Co. The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material (e.g., NAFION.TM.) intermingled with the catalytic and carbon particles, or (2) catalytic particles, sans carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder. One such MEA and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993, and assigned to the assignee of the present invention.
The MEA is sandwiched between sheets of porous, gas-permeable, conductive material, known as a "diffusion layer", which press against the anode and cathode faces of the MEA and serve as (1) the primary current collectors for the anode and cathode, and (2) mechanical support for the MEA. Suitable such primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh noble metal screen, and the like, through which the gas can move to contact the MEA, as is well known in the art.
The thusly formed sandwich is pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors and conducting current between adjacent cells (i.e., in the case of bipolar plates) internally of the stack, and externally of the stack in the case of monopolar plates at the ends of the stack. The secondary current collecting plates each contain at least one so-called "flow field" that distributes the fuel cell's gaseous reactants (e.g., H.sub.2 and O.sub.2 /air) over the surfaces of the anode and cathode. The flow field includes a plurality of lands which engage the primary current collector and define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply manifold at one end of a flow channel and an exhaust manifold at the other end of the channel. Serpentine flow channels are known and connect the supply and exhaust manifolds only after having made a number of hairpin turns and switch backs such that each leg of the serpentine flow channel borders at least one other leg of the same serpentine flow channel (e.g., see U.S. Pat. No. 5,108,849).
The pressure drop between the supply manifold and the exhaust manifold is of considerable importance in designing a fuel cell. One of the ways of providing a desirable pressure drop is to vary the length of the flow channels extending between the supply and exhaust manifolds. Serpentine channels have been used heretofore to vary the length of the flow channels. Serpentine channels are designed to allow some limited gas movement between adjacent legs of the same channel via the diffusion layer so as to expose the MEA confronting the land separating the legs to reactant. In this regard, gas can flow from an upstream leg of the channel (i.e. where pressure is higher) to a downstream leg of the same channel (i.e. where gas pressure is lower) by moving through the diffusion layer over/under the land that separates the upstream leg from the downstream leg of the flow channel. However, when the legs of a channel are too long, an excessive pressure drop can occur between adjacent legs of the same flow channel or between the ends of the legs (i.e. where they turn to adjoin the next adjacent leg) and/or an adjacent supply or exhaust manifold. Such excessive pressure drop can, in turn, result in the gaseous reactant excessively short circuiting between the adjacent legs, or ends and manifolds, rather than flowing through the full length of the channel. Such flow is considered to be excessive when it exceeds the amount of reactant that can be reacted on the MEA confronting the land between the legs .